Supported catalysts, endowed with high activity in the polymerization of alpha-olefins are known and are obtained by the reaction of an Al-alkyl compound with catalyst components comprising a Ti compound, in particular TiCl.sub.4, and an electron-donor compound supported on a Mg dihalide in active form.
The Mg dihalide in active form shows halos in the X-rays powder spectrum in the place of the most intense diffraction lines which appear in the powder spectrum of the normal Mg dihalide.
Examples of such catalysts are disclosed in U.S. Pat. Nos. 4,107,413; 4,107,414 and 4,226,741.
A peculiar feature of the supported catalysts endowed with higher stereospecificity and activity in the polymerization of the alpha-olefins is that of being prepared according to methods which lead to the formation of components comprising the Ti compound supported on the magnesium dihalide, which do not contain or contain only low percentages of Ti compounds extractable in hot conditions with boiling heptane and with TiCl.sub.4.
The preparation of these catalysts is generally carried out in conditions in which the solid comprising the magnesium dihalide in active form is reacted in suspension in the Ti compound; the excess of Ti compound is then removed by filtration at temperatures at which no Ti compounds extractable with boiling heptane and with TiCl.sub.4 at 80.degree. C. remain on the solid; the solid is then repeatedly washed with hydrocarbon solvents to remove any traces of soluble Ti compound.
According to other methods, the Mg dihalide, the electron-donor compound and the Ti compound, optionally in the presence of grinding coadjuvants, are co-milled until activation of the magnesium dihalide, i.e. until appearance, in the X-rays powder spectrum of the Mg dihalide, of halos in the place of the most intense lines which appear in the powder spectrum of the normal magnesium dihalide.
The solid is then subjected to a hot treatment with a hydrocarbon solution of a halogenated hydrocarbon compound and the liquid phase is separated by hot filtering in order to remove the soluble compounds. The solid is then further washed with an aliphatic hydrocarbon.
Examples of these methods are described in published European Patent Application Nos. 0004789 and 0004791.
According to another method, the solid catalyst component obtained by co-milling a Mg dihalide and an electron-donor compound and by successive treatment with an excess of TiCl.sub.4 in hot conditions is further treated in suspension in a halogenated hydrocarbon solvent, is separated from the liquid phase and finally repeatedly washed with a hydrocarbon solvent (U.S. Pat. No. 4,146,502).
All the solid catalyst components obtained according to the methods mentioned hereinabove do not contain, or contain only small percentages of Ti compounds not extractable with heptane and with TiCl.sub.4.